In vitro studies using the plasma clot culture system have been performed in order to compare the red cell pregenitors able to rise to erythrocytic colonies in 7 days (CFUE) in the bone marrow of polycythemia vera (PV), secondary polycythemias and normal subjects. In PV but never in normal individuals or secondary polycythemias, the bone marrow cells producing erythroid colonies without addition of erythropoietin were found. The erythropoietin dode response curves in PV is biphasic with a plateau up to a concentration of erythropoietin of 0.02-0.05 i.U./ml followed by a near normal response to erythropoietin at higher doses. Thus our results demonstrate that two populations of erythroid stem cells coexist in PV, one being abnormally sensitive to (or independant of) erythropoietin, the other normally responding to erythropoietin. After remission induced by P32 treatment, the abnormal population can disappear but the prognostic significance of this disappearance is uncertain. On the whole these results are in agreement with those of others laboratories using the plasma clot culture system. The reasons of the disagreement with the data published using the methylcellulose technic of culture are discussed.